Surface Tension Wetting

Solder Chemistry

Chemical additives in the flux are necessary to reduce the surface energy of the molten alloy. This action allows the liquid solder to flow and coat the pads instead of forming isolated beads. Active flux ensures complete spreading on the copper surfaces during reflow.

Process Variable

Temperature and surface cleanliness are the primary factors that influence how well the liquid metal flows. Elevated reflow temperatures lower the surface tension of the molten solder, which improves its ability to wet the metal surfaces. However, excessive heat can accelerate oxidation of both the solder and the board pads, reversing this beneficial effect.

Balancing the temperature and the conveyor speed ensures that the wetting action completes before the solder begins to solidify.

Assembly Failure

Incomplete flowing of the liquid solder creates joints that are fragile and prone to failure under mechanical stress. When the wetting forces are inadequate, the solder may form a non-wetted joint where it sits on the pad without creating a true intermetallic layer. This defect is particularly dangerous because the board may pass initial testing but fail when the product is exposed to vibrations or temperature changes in the field.

Resolving these wetting issues during prototype testing ensures the durability of the final product.